docs: def get_occupied_slots(instance):
      """
      Return a list of slots for which values have been set.
  
      (While a slot might be defined, if a value for that slot hasn't
      been set, then it's an AttributeError to request the slot's
      value.)
      """
      return [slot for slot in get_all_slots(type(instance))
              if hasattr(instance,slot)]